Magnetic North III dependence of FMR measurements NiFe ......4 Easy 0)(0 Hard(900) See review...
Transcript of Magnetic North III dependence of FMR measurements NiFe ......4 Easy 0)(0 Hard(900) See review...
Angular dependence of FMR measurements in exchange coupled
NiFe/NiO bilayers: Experiment and Theory
B.W. Southern*N. Grenda*#, P. Hyde*, Y.S. Gui*, M.P. Wismayer*,
C.‐M. Hu*, K.‐W. Lin&, and J. van Lierop*
*Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, Canada
# Institut fur Angewandte Physik und Zentrum furMikrostrukturforschung, Universitat Hamburg, Hamburg, Germany
& Department of Materials Science and Engineering, National Chung HsingUniversity, Tiachung,Taiwan
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Magnetic North III
Outline
• Introduction • Experimental results• Theoretical Interpretation• Summary
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• Ferro (F) and Anti‐ferro (AF) films in contact exhibit the phenomenon of exchange‐bias first observed by Meiklejohnand Bean (1956) in CoO
• Usually characterized by a shifted hysteresis loop after field cooling.
• Attributed to a unidirectional anisotropy
• Applications in information storage technologies and was first practical application of AF in spin valves
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770K
Cooled in 1T
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Easy (00) Hard(900)
See review articles by• Nogues and Schuller JMMM 192,203(1999)• Berkowitz and Takano JMMM 200,552 (1999)• Stamps J. Phys. D. 33, 247 (2000)• Kiwi JMMM 234,584 (2001)
Hc=( hR‐hL)/2Hex=(hR+hL)/2
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• Different experimental techniques yield different values for the coupling field Hex at the interface
• Hysteresis loops involve irreversible switching of the magnetization
• FMR and BLS generally involve small pertubations of the magnetization about equilibrium and are reversible
• In this talk I will discuss a particular NiFe/NiO bilayer systemwhere both magnetization and FMR measurements have been performed by research groups in Manitoba.
• The bilayer was prepared by the group of Ko‐Wei Lin at the National Chung Hsing University in Taiwan using a dual ion‐beam sputtering technique at room temperature. Lin et al Appl. Phys. Lett. 100, 122409(2012)
• The NiFe/NiO film was fabricated on a SiO2 substrate and no external field was applied during any stage of the growth
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epoxy
SiO2
Fabrication
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• NiO film layers had crystallitesizes between 5 and 20 nm indiameter that penetrated the16 nm thickness (XRD)
• ZFC and FC DC susceptibilitymeasurements indicated ablocking temperature TB=360K
• NiO has a cubic fcc structure with alternating ferromagneticsheets along the [111] direction
• Since no field was applied during fabrication, many AF domain configurations could be present.(there 4 possible [111]directions as well as perhapsthree in each plane)
Analysis
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McCord: Adv in Solid State Physics 48, 157 (2009)
• The presence of exchange bias and coercivitydepends on the thickness of the AF layer
• For 16 nm thickness, Hc and Hex could be of the same order of magnitude with Hex larger
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McCord: Adv in Solid State Physics 48, 157 (2009)Easy
Hard
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• Magnetometry measurements by Van Lierop’s group on a sampleof length 6 mm and width 2 mm for both a single and bilayer wereperformed
• Films were heated to 400K and cooled in a field of 2T
• Single NiFe layer exhibited negligble coerciviity and no exchange bias
• Bilayer sample exhibited both coercivity and exchange bias shift Hex ~ 2.0 mT along the cooling field direction
• Hc ~ 1.1 mT @ 300K
Magnetometry
NiFe
NiFe/NiO
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FMR Measurements
• FMR measurements on the same samples were performed by the group of Can‐Ming HuNiFe: L=21 mm W=20.0 mm T=9nmNiFe/NiO:L=16.3 mm W=13.8 mm T=9 nm
• Broadband microwave absorption spectroscopy was used to detectthe FMR frequency as evidenced by a transmission minimum
• A reference measurement at 150 mTis used to perform a subtraction at lower fields
30mT
150mT
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FMR
• External field is applied in‐plane atvarious angles φ with respect to thelong direction
• For each φ, a reference measurementis made at ± 150 mT and then either an up sweep or a down sweep is carried out between ±30 mT at intervals of 0.4 mT
• No heating or cooling in a field performed• Two distinct types of behaviour are observed for the FMRfrequency as a function of the field which depend upon thedirection of the field.
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• Irreversible switching is observed in the range ‐6mT < H < 6mT• For all angles < 450 a single peak is found• Frequency range is 2.0 < f < 2.5 GHz
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• Irreversible switching is observed in the range ‐8mT < H < 8mT• For all angles >600 a two peaks are found• Suggests a two step reversal process• Frequency range is 2.7 < f < 3.4 GHz
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0
2r sf H M H
μ0Ms=963 mT, γ= 176 GHz/T
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• For 4.5 GHz we can plot the resonant field Hr as a function of φ• Can be fit to the form Hr= a0 + a1 cos(φ) + a6 cos(6φ)a0= 25.306 mT, a1= .345 mT and a6=‐0.133 mT for up sweep (a)a0= 25.338 mT, a1= .296 mT and a6=‐0.237 mT for down sweep (b)
• Suggests that there may be a unidirectional anisotropy as well asa six‐fold anisotropy
Up sweep Down sweep
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? Theory
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• In order to model this behaviour we treat the NiFe as a single domain and include the effect of the NiO as introducing anisotropies at the interface
• The energy of the system can be written as
2 2 2 2 2 2 2 22 4
6 6 4 2 2 46
1.2 2 4
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z x x y y z z x
x y x y x y x
x xE H M M S S S S S S S
x S S S S S S jS
• Use spherical coordinates to find extrema of E as function ofthe field direction φH and magnitude
• Calculate FMR frequencies using Smit‐Beljers formula (1955)
21sins
E E EM
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90 degrees x2=3 mT x4= ‐8mT x6= 0.1 mT
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0 degrees x2=3 mT x4= ‐8mT x6= 0.1 mT
210 degrees 90 degrees
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45 60 80
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Downsweep90 degrees
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Downsweep0 degrees
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• Bilayers were fabricated in zero field and no field cooling wasperformed for the FMR measurements
• Minor hysteresis loops appear as the field direction is rotatedfrom 0 to 90 degrees with respect to the longer sample side
• The single domain model is too simple to make detailedcomparisons with experiment but does indicate thatanisotropies contribute to the angular dependence of theresonant frequencies
• F layer may not be homogeneous at interface• Micromagnetic results indicate that reversal is not single domain• AF layer is treated as being static and providing an effectiveanisotropy
• The micromagnetic calculations need to be extended to multiplelayers which would allow some dynamics in the AF
Summary
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The End